Grounded-grid cathode-driven triggered waveguide switch



April 22, 1969 GOLDlE ET AL 3,440,486

GROUNDED-GRID CATHODE-DRIVEN TRIGGERED WAVEGUIDE SWITCH PRIOR ART FIG.|.

GRID SPIKE FIGS.

qlTTER T|ME GROUNDED- CATHODE OPERATION UNFIRED GRID (VOLTAGE IOOV/CM) FIRED GRID VOLTAGE ARC CURRENT (lOAMPS/CM) 4 B F E E M .M. S T O M N E L I l L G E N E T M 0 N O O E H HE R TE T6 R AG AA U m CT C DL C E0 v R RV m A H F N U GROUNDED GRID OPERATION INVENTORS ,Herber'r W Cooper an Lox vson FW ATTORNEY WITNESSES Harry Goldie United States Patent ()flice 3,440,486 Patented Apr. 22, 1969 U.S. Cl. SIS-39 4 Claims ABSTRACT OF THE DISCLOSURE The triggered waveguide switch, hereinafter referred to as TWS, was developed as a grounded-cathode drivengrid tube and it was operated in a manner similar to that of an ordinary hydrogen thyratron. A triggered waveguide switch employs a thyratron-type of electron discharge having the usual cathode and anode but with a section of waveguide sealed to the envelope and serving as the control grid of the electron discharge device. The section of microwave guide which serves as the grid is provided with appropriate pressure windows scaled across the walls of the waveguide and the broad walls of the waveguide section have suitable perforations therein to permit the passage of electrons from the cathode to the anode. When operated as a transmission line component it was isolated by two insulating spacers located between the connecting flanges. Since it was not operated at a common potential with the waveguide transmission line serious disadvantages limited the usefulness of the TWS. Since the waveguide section was operated above ground potential a large grid-to-grid ground capacitance caused waveform distortion and a decrease of the voltage of the trigger signal. Another consequence of the large gridground capacitance is a large current spike which appears across the trigger generator at commutation. This creates the need for a grid de-spiking network. The microwave flanges had to be separated by an electric insulator impervious to voltage puncture by the total voltage of the spike superimposed on the trigger pulse. The insulator can lead to RF flange leakage; another deleterious effect of the insulator is the degradation of the slope of the leading edge of the trigger pulse. This decrease due to the flange capacitance led to excessive anode commutation jitter. Herein is disclosed a conventional triggered waveguide switch operating in a unique grounded-grid cathode driven circuit configuration which avoids the necessity for electrically isolating the microwave section grid from the remainder of the waveguide transmission line.

This invention relates to a waveguide switch and more particularly to a triggered waveguide switch in which a section of waveguide transmission line serves both to propagate microwave energy and at the same time serves as the grid for controlling the development of the plasma in the thyratron-type electron discharge device.

Heretofore, triggered waveguide switches were operated as grounded-cathode driven-grid tubes similar to ordinary hydrogen thyratrons and the section of the waveguide serving as the grid had to be electrically isolated from the waveguide transmission line which created several problems, such as insulation problems, radio frequency flange leakage and wave distortion of the triggering pulse.

The present invention is directed to a circuit for operating the triggered waveguide switch as a grounded-grid cathode-driven electron discharge device in which the section of waveguide serving as the grid of the device is electrically connected to and maintained at the same potential as the portions of the waveguide transmission line.

From the above it is apparent that it is an object of the present invention to provide an improved circuit arrangement for operating a triggered waveguide switch in such a manner that the section of the waveguide serving as the grid can be operated at a potential common with that of the waveguide transmission line.

Other and further objects will become apparent from the following description or in connection with the accompanying drawings, in which:

FIGURE 1 is a circuit diagram illustrating the prior art;

FIG. 2 is a circuit configuration of the triggered waveguide switch in accordance with the present invention;

FIG. 3 is a graphical illustration of an oscilloscope display of a prior grounded cathode operation; and

FIG. 4 is a graphical illustration of the oscilloscope display or grounded grid operation in accordance with the present invention.

Referring to the drawing, and particularly to FIG. 2, a transmission line through which microwave energy may be propagated in the dominant TE10 mode is illustrated at 10. As illustrated, it is readily apparent that it includes at least three sections, one of Which, illustrated at 11, serves as the grid for a hydrogen thyratron type electron discharge device 12 which includes an envelope 13 sealed to the sides of the waveguide section 11. In order to complete the envelope the microwave guide section 11 has suitable pressure windows 14 and 16 which seals the opening in the waveguide section. The pressure windows are provided with suitable microwave irises in order to permit the propagation of microwave energy through the waveguide section.

The complete envelope formed by the toroidal glass section 17, sealed to the outer walls of the waveguide section 11 and the pressure windows 14 and 16 sealed across the waveguide section, enclose and contain the hydrogen atmosphere in the cathode 18 and anode 19 and the grid component 21. The latter is in the form of the perforated side walls of the waveguide section 11. The perforations in the side walls are indicated at 22. The anode 19 is connected to a source of positive potential represented by the terminal 20.

The waveguide section 11 is provided with the usual flanges 24 which are mechanically coupled to similar flanges 26 and 27, respectively, of the other sections of the waveguide. It is to be understood that the flanges on the section 11 have a very low impedance connection to the flanges 26 and 27 so that there is no potential difference between the waveguide section 11 and the other sections of the waveguide transmission line which is grounded. The purpose of this is to avoid any grid-toground capacitances, radio frequency flange leakage, trigger waveform distortion and the requirement for a grid de-spiking network. The flanges on all the other sections of the transmission line are also at the same potential. This connection also makes certain that no noisy electrical connection exists between the flanges. It is essential that the waveguide section 11 be at all times at a com- {non potential with the other section of the transmission Referring now to FIG. 1, illustrating the prior art, it is to be noted that the waveguide section 31 which corresponds to waveguide section 11 of FIG. 2 is electrically insulated, by insulating spacers 32 and 33, from the flanges of the other sections of the transmission line. This is essential since the cathode 34 is connected to ground through the direct current connection 36. Since the other two sections of the transmission line 37 and 38 are also grounded it is apparent that there is substantial capacity to ground designated C between the flanges of the waveguide section 31 and the sections 37 and 38. Also, in parallel with this, is the capacitance, indicated at C between the waveguide section 31 and the grounded cathode connection 36. Also shown is the additional capacitance between the anode and the electrically isolated waveguide section 31 designated at C The particularly deleterious capacitance of the construction of FIG. 1 is the capacitance C between the flanges of the waveguide section 31 and those of waveguide transmission line because this constitutes a capacitance in parallel to the section 31, serving as the grid of the thyratron, and the cathode. This capacitance adds to the usual capacitance between the capacitance C between the cathode and grid of a thyratron tube. Because of this extra capacitance in the prior art arrangement it was necessary to provide a despiking network 39 between the trigger pulse generator 41 and he waveguide secion 31.

The driven-cathode arrangement of the present invention, as illustrated in FIG. 2, eliminates the capacitance C between the flanges of the waveguide sections, thus eliminating the need for the de-spiking network 39 between the trigger generator and the grid element of the thyratron device. This is accomplished by electrically connecting together the flanges of the sections of the waveguide transmission lines and providing a high impedance AC path between the cathode 18 and the ground G through a resistor R which has connected in parallel therewith a unidirectional impedance in a form of a diode D.

The impedance of the resistor R should be very high as compared to the internal impedance R of the trigger pulse generator 42 that is, several times the generator impedance when the tube is commutating, so that it will not heavily load the output of the trigger pulse generator. It will be noted that the anode 43 of the diode D is connected to the cathode 18 of the triggered waveguide switch and therefore a negative trigger pulse from generator 42 will automatically put the diode in its high impedance state.

Whereas the trigger generator 41 in FIG. 1 must provide a positive pulse to the output terminal 46 in order to fire the TWS in FIG. 1, the trigger generator 42 of FIG. 2 supplies a negative trigger pulse at terminal 47 and across the resistor R. Under these conditions, the diode D is open and therefore does not dissipate any of the energy of the negative trigger pulse supplied from terminal 47. However, it will be apparent that as soon as the thyratron device 12 of FIG. 2 commutates, as a result of this same trigger pulse applied to the TWS cathode 18, the instantaneous starting of electron current between the cathode 18 and the anode 19 will cause the anode 43 of the diode D to swing positive and instantaneously close. This rapidly lowers the resistance in the trigger circuit between the cathode and the anode to less than one ohm. The diode then carries the TWS current for the remainder of the commutation interval. The advantages of the present invention over the prior art is illustrated by comparison of FIGS. 3 and 4. It will be noted that in FIG. 3 with the prior art circuit there is substantial arc current jitter whereas within the circuit arrangement of the present invention the arc current jitter is substantially reduced. The fact is that the capacity effect of the prior art circuitry does disturb the triggering pulse and jitter in the microwave application of the thyratron switch.

In the operation of the device a negative trigger pulse .4 is supplied from the terminal 47 of the trigger generator 42 for a duration of several microseconds and is applied to the cathode 18. With the diode D in the non-conducting state the trigger generator 42 sees a reasonably matched load in the resistor R. When the cathode-grid potential reaches a critical level due to the trigger, the electron discharge device 12 commutates and an are current flows in the anode cathode circuit. This reverses the voltage across the diode D causing it to conduct thus short circuiting the cathode resistor R. The anode 19 draws its current only from the cathode 18 since it is a thermionic source and it does not draw current from the grid.

The advantages of the grounded-grid cathode-driven circuit configuration is as follows:

(A) The waveguide flange capacitance is eliminated thus removing a need for a de-spiking network. This eliminates trigger waveform distortion and allows the trigger source to work into a matched load.

(B) The waveguide of the TWS is now at common potential with the transmission line.

(C) Danger of RF leakage at the flanges is removed since direct metal contact is now made.

(D) Jitter characteristics in the arc current have been significantly improved.

We claim as our invention:

1. A microwave switch comprising an electron discharge, having a cathode, anode and a control element, said control element being in the form of a section of waveguide adapted to support the propagation of electromagnetic wave energy and to form part of a grounded waveguide transmission line, said control element section being ohmically connected to said transmission line and having aligned apertures in its sidewalls to permit electron streams to flow from cathode to said anode to create a dense plasma forming a radio frequency barrier when said electron discharge device is activated to the conducting condition, a cathode resistor between said cathode and ground, a source of positive potential connected to said anode, a diode connected across said resistor, said diode being poled so that it is open when said cathode is positive with respect to ground and means for supplying a negative biasing potential across said resistor to bias said electron discharge device to conducting condition.

2. The combination as set forth in claim 1 in which said diode has its anode connected directly to the cathode of said electron discharge device.

3. The combination as set forth in claim 1 in which the impedance of said resistor is very high as compared to the internal impedance of said source of biasing potential.

4. The combination as set forth in claim 2 in which said source of negative biasing potential is a source of triggering pulses for controlling the conduction intervals of said electron discharge device.

No references cited.

HERMAN K. SAALBACH, Primary Examiner.

S. CHATMON, JR., Assistant Examiner.

US. Cl. X.R. 

